A Sport-season friend is somewhat like a fair-weather friend, except you're only friends with them during/because of a certain sport season. They mostly come in the form of teammates, coaches, or fans. Sport-season friends are usually people that you would not have otherwise gotten to know, so integrating them into your primary group of friends is often difficult. While the friendship occasionally extends into other parts of the year, it is at its strongest during the sports season. They are very prominent among high school athletes--especially when there are age/social rank differences.
Kid 1: Hey, there goes Jesse. Why didn't you say "hi"?
Kid 2: Well, he's just a sport-season friend, anyhow.